1. Field of the Invention
This invention relates to a stroke control circuit for a variable displacement reciprocating piston pump where the pump is destroked by increasing the pressure in the pump crank case to directly bias the radial pistons and, in particular, to a valve and orifice for controlling a radial piston pump.
2. Description of the Prior Art
The displacement of a radial piston pump is controlled by regulating the fluid pressure in its stroke control chamber or crankcase. This concept is taught in U.S. Pat. No. 3,002,462 issued to Raymond. As explained in the Raymond patent, increasing the pressure in the crankcase will destroke the piston and reduce displacement of the pump. Numerous methods for controlling the addition or withdrawal of pressurized fluid from the crankcase are disclosed in the prior art. U.S. Pat. No. 3,002,462 shows several arrangements for routing high pressure oil from the output of the pump through restriction orifices and back to the crankcase for destroking the pump in response to pump output and pressure drop through the restriction orifices. U.S. Pat. No. 3,526,468 issued to Moon shows a circuit for controlling the stroke in one pump of a two pump circuit using pump outlet pressure or pump inlet pressure regulated by a control valve. Although these and other control systems enable the pump to operate, the arrangement and function of the radial piston pump provides difficulties that detract from its operation and are not compensated for by known control circuits.
These control problems stem from the arrangement and characteristics of the radial piston pump. Controlling displacement of the pump by decreasing or increasing the pressure in the pump crank case to bias the radial pistons inward or outward and effect stroking or destroking increases the difficulty of control since it varies the capacitance of the stroke control chamber or crankcase. This variation in capacitance makes control nonlinear. In addition, response time to control signals is delayed by the amount of time necessary for the eccentric drive of the pump to push the pistons out after an increase in chamber pressure. Aside from the delay, pushing the pistons out to destroke the pump causes pressure spikes in the pump output. Finally, leakage from the piston chambers to the crankcase is usually high when the pump is at maximum power conditions. Thus the control system typically has an orifice for relieving crankcase pressure which is sized on the basis of the high leakage conditions at maximum power output. This results in a large energy loss at low power or standby conditions when a high quantity of control flow is needed to compensate for the low piston leakage at reduced power conditions.
Accordingly, it is an object of this invention to provide a control circuit that will reduce the detrimental effects of the stroking-destroking nonlinearity and destroking delay.
It is a further object of this invention to provide a control circuit that will limit spikes in output pressure when destroking of the pump.
Another object of this invention is to provide a control circuit minimizing energy loss.